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Depending whether you stay with Borland or not.
For C++ Builder you only have to change the Delphi grammar to the C++ one and perhaps look if all cases of variables and object functions are in C++ style.

String literals in C use double quotes. Is the problem perhaps the concatenating of strings you are doing in the call to FindWallpaper?
dir + "\\" + sr.Name
should be the first argument. Not sure about
dir + '\' + sr.Name
What type is your first argument to FindWallpaper? string or AnsiString?

A small addition to my conversion hints:
&& is the boolean and operator and
& is the bitwise and operator.

And a comment on andrewjb
TStringList files will put the value on the stack unlike DELPHI which does everything by reference. It should work as well but you won't see anything in the list after the function has finished since only the local copy was filled with file names.

You can't use just 'TStringList files', because Borland C++ Builder doesn't let you create VCL-derived components (like TStringList) on the stack. You HAVE to create them on the heap - hence the error message.